Piston ring construction

ABSTRACT

A two-part compression ring assembly which consists of interlocking channel shaped ring members wherein one interlock flange of each member is received in the groove of the other. The interlock flanges have confronting annular faces beveled at an angle of about 10* and provided with a clearance space of about 0.002 inch. A three-part compression ring assembly includes an expansion ring of L shaped cross section having radial spring fingers for urging the remainder of the assembly toward one piston groove wall and having a slotted cylindrical flange for radial flexibility. A key for preventing relative rotation of the interlocking ring members is provided on one ring member visible from the exterior periphery of the ring to be received in appropriate notches at the ends of the mating ring.

United States Patent 11 1 Bergeron Feb. 19, 1974 PISTON RING CONSTRUCTION [76] Inventor: Alfred Bergeron, 1288 Bagley, Lot

6, Dallas, Tex. 75211 [63] Continuation-impart of Ser. No. 66,992, Aug. 26,

1970, abandoned.

52 us. Cl. 277/193 [51] Int. Cl Fl6j 9/16.

[58] Field of Search.... 277/192, 193, 195, 197, 199

Primary ExaminerWilliam T. Dixson, Jr.

Assistant Examiner-Robert L. Smith Attorney, Agent, or Firm-Cecil L. Wood; Peter J. Murphy [57] ABSTRACT A two-part compression ring assembly which consists of interlocking channel shaped ring members wherein one interlock flange of each member is received in the groove of the other. The interlock flanges have confronting annular faces beveled at an angle of about 10 and provided with a clearance space of about 0.002 inch. A three-part compression ring assembly includes an'expansion ring of L shaped cross section having ra- [56] References Cited dial spring fingers for urging the remainder of the as- U TED STA S PATENTS sembly toward one piston groove wall and having a 2,594,282 4 1952 Bergeron 277 197 Slotted Cylindrical flange for radial fiexibilitykey 2,670,258 2/1954 Bergeron 277/197 for preventing relative rotation of the interlocking ring 2,670,255 2'/ 1954 Bergeron 277/193 members is provided on one ring member visible from 3,545,775 12/1970 Prasse 277/216 th exterior periphery of the ring to be received in ap- FOREIGN PATENTS 0R APPLICATIONS propriate notches at the ends of the mating ring.

123,352 2/1929 Great Britain 277/193 10 Claims, 16 Drawing Figures Ii 49 l 1 1 I lliliill' l PAIENIEDFEB 1 9mm 3,792.86?

' sum 1 or 2 7 l3 3/ M *F 3 3; L I

I" "ll 1 IHIII- 22 25 Fig.2

INVENTOR Alfred Bergeron.

Fig.8 BY gmW/M/ ATTORNEY PISTON RING CONSTRUCTION REFERENCE TO RELATED APPLICATION BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to piston rings, and is particularly concerned with improvements in piston rings of the two-part interlocking type including two-part compression rings and combination compression and oil jcants prior US. Patents identified as follows: No.

2,190,204, issued Feb. 13, 1940; No. 2,594,282, issued Apr. 29, 1952; No. 2,670,257 issued Feb. 23, 1954, and No. 2,670,258 issued Feb. 23, 1954. The piston rings described in applicants prior patents and the piston rings according to the present invention are particularly adapted for use in internal combustion engines.

An object of this inventionis to provide a ring consisting of interlocking ring members and having an improved interlockingjoint between the ring members for improved radial and longitudinal sealing between the members.

Another object of this invention is to provide a piston ring assembly consisting of interlocking ring members and having improved means for preventing relative rotation of the ring members.

A further object of this invention is to provide a piston ring assembly improved means for maintaining the piston ring against one side of the piston ring groove thereby preventing hammering of the walls of the groove by the ring. I

A still further object of this invention is to provide an improved piston ring assembly including an improved ring expander for conforming the piston ring to variations in the cylinder wall surface.

These objects are accomplished in a piston ring assembly comprising inner and outer interlocking split ring members each having upper and lower radial flanges and an interconnecting web defining an annular groove. One flange of eachring member defines an interlock flange dimensioned to be received in the groove of the mating ring member. The interlock flanges have confronting annular seal surfaces beveled at an angle of approximately relative to the plane of the ring assembly; and in assembledrelation a clearance within the range of from about 0.001 to about 0.002 inch is provided between these beveled surfaces. The assembly may include van expansion ring having radially extending spring fingers for urging the remainder of the assembly longitudinally against one wall of the piston groove, the expansion ring including a cylindrical flange having angularly spaced slots to resiliently urge the remainder of the assembly radially into engagement with the cylinder wall. The assembly may include a key for preventing relative rotation between the interlocking ring members, which key is provided on one member visible from the outer periphery thereof for engagement by the ends of the interlocking member.

The novel features and the advantages of the invention, as well as additional objects thereof, will be understood more fully from the following description when read in connection with the accompanying drawings.

DRAWINGS FIG. 1 is a plan view of a two-part compression ring assembly, in condition to be assembled with a piston;

FIG. 2 is a fragmentary edge view of the ring assembly as viewed along the line 2-2 of FIG. 1; 7

FIG. 3 is a fragmentary plan view of one joint of the assembly of FIG. 1, as compressed in a cylinder piston v assembly;

FIG. 4 is a fragmentary perspective view of one joint of the ring assembly in FIG. 1;

FIG. 5 is a fragmentary bottom view of the ring assembly portion illustrated in FIG. 2;

FIGS. 6 and 7 are transverse sectional views through the ring assembly taken along the indicated lines in FIGS. 1 and 2, respectively;

FIG. 8 is a fragmentary perspective view of a compression ring assembly similar to that of FIG. 1, and including an auxiliary sealing and expander ring;

FIG. 9 is a fragmentary. sectional view of a pistonpiston ring assembly, taken in a radial plane, illustrating the compression ring assemblies of FIGS. 1 and 8;

FIG. 10 is a plan view of a compression-oil ring assembly according to the invention;

FIG. 11 is an edge view of the ring assembly of FIG. 10;

FIG. 12 is a fragmentary plan view of one joint of the ring assembly of FIG. 10 in compressed condition;

FIG. 13 is a fragmentary perspective view of the assembly of FIG. 10 illustrating the sealing joint of the compression ring member;

FIGS. 14 and 15 are transverse sectional views through the ring assembly taken along the indicated lines in FIG. 10; and

FIG. 16 is a transverse sectional view, generally similar to FIG. 14, illustrating an alternative form of compression-oil ring assembly.

DESCRIPTIONOF THE PREFERRED EMBODIMENTS For purposes of orientation, the illustrations of the several piston ring assemblies are illustrated for assembly with a piston for an internal combustion engine wherein it is assumed that the piston head faces upward or toward the viewer, and that therefore the assemblies would move either downward or into the paper on the 1 power stroke of the associated piston for example. An

exception is FIG. 5 which illustrates the bottom face of the ring assembly.

In the drawings there are shown several forms of piston ring constructions'or assemblies which embody the features of the invention. FIGS. 1 through 7 illustrate a compression ring assembly which consists of two interlocking split ring members. FIGS. 8 and 9 illustrate a compression ring assembly similar to that of FIGS. 1 through 7, but including an additional spring pressure ring member. FIGS. 10 through 15 illustrate a compression-oil ring assembly consisting of two interlocking ring members with improved key means for preventing relative rotation of the ring members. FIG. 16 illustrates an alternative form of oil ring assembly.

Embodiment of FIGS. 1 through 7 The piston ring structure illustrated in FIGS. 1 through 7 is a two-part compression ring assembly 10 consisting of inner and outer interlocking split ring members 11 and 12. The inner ring member 11 includes upper and lower radial flanges 13 and 14 interconnected by a cylindrical web portion 15 which defines the inner wall of the ring member, with the ring member defining an outward facing annular groove having a cylindrical base 16. The outer ring member 12 includes upper and lower flanges and 21 respectively interconnected by a generally cylindrical web portion 22 defining the outer cylindrical wall for the ring member, the ring defining an inward facing annular groove having a cylindrical groove base 23.

All of the flange faces are machined to define flat surfaces lying in planes perpendicular to the axis of the ring assembly, with the exception of the upper face of the flange 14 and the lower face of the flange 20.

In assembled relation, as best seen in FIG. 5, the lower flange 14 of the inner ring is received within the annular groove of the outer ring, and the upper flange 20 of the outer ring is received within the annular groove of the inner ring; and these flanges l4 and 20 are therefore referred to as interlock flanges. These interlock flanges are provided with peripheral cylindrical surfaces which are dimensioned to seat on the respective cylindrical base surfaces 16 and 23 of the annular grooves, so that when the ring members are assembled together there is no clearance between these cylindrical surfaces. The side faces of the ring member flanges are flat, lying in planes perpendicular to the 'axis of the ring assembly, with the exception of the confronting faces of the interlocking flanges l4 and 20 which are beveled relative to a perpendicular plane. These beveled surfaces are beveled at an angle of approximately 10 relative to the plane of the ring assembly, a bevel angle of 10 having been found very satisfactory. When the ring members are assembled and compressed together both axially and radially to eliminate clearance between the confronting flat and cylindrical surfaces, a clearance 24 of about 0.0002 inch is provided between the confronting beveled surfaces of the interlocking flanges, preferably no less than 0.0001 inch. This clearance eliminates binding between the ring members 11 and 12 which could hinder proper expansion of the ring assembly.

When the ring members 11 and 12 are assembled together, in condition to be installed in a piston-cylinder assembly, the ring members joints are normally disposed diametrically opposite from each other, as illustrated in FIG. 1, and the ring members include coacting means to be described for preventing relative rotation to maintain this oppositely spaced position of the joints. Also, the joints are in expanded condition in FIGS. 1 and 4, and when placed in a piston-cylinder assembly the rings are axially compressed in the usual manner wherein the joints are closed as indicated in FIG. 3. In this compressed condition, the inner ring will urge against the outer ring to produce the relationship indicated in FIG. 9.

Over a period of extended use, there will, be wear on all ofthe surfaces of the rings,.inc'luding the interior cylindrical sealing surfaces; and the clearance 24 prov vided on the beveled surfaces of the interlocking flanges will permit the inner and outer rings to move together as wear occurs to maintain the desired sealing between the interlocking ring members.

FIG. 4 is a perspective view illustrating the joint between the ends of the inner ring member 11, shown at the bottom of FIG. 1, wherein the inter-engaging ends of the ring member are formed with tongues 26 and 27 coacting with each other and with the ring member 12 to provide a seal both radially and longitudinally of the pistons to which it is assembled. As best seen in FIG. 4 the tongues are provided essentially in the upper flange 13 of the inner ring, each tongue having a radial width approximately one half that of the flange 13. The tongue 27 is an outer tongue, lying at the outer edge of the flange 13, while the tongue 26 is a mating inner tongue. When the ring members are radially compressed, as seen in FIG. 3, the tongues overlap radially to provide a seal obviating radial blow-by of the compressed gases. As viewed from the top of the ring assembly, the confronting cylindrical surfaces defined by the overlapping tongues are radially spaced from the inner edge of the flange 20 of the outer ring 12 so that these vertical surfaces are stepped radially to obviate longitudinal blow-by through the ring assembly.

A similar joint is provided in the outer ring member 12 including tongues 26a and 27a in the lower flange 21, as indicated at the top of FIG. 1; however, this joint includes means coacting with means on the inner ring member 11 for preventing relative rotation of the ring members. As best seen in FIGS. 2, 5 and 7, an integral key 30 is provided on the inner ring 11 and is disposed diametrically opposite from the joint for the inner ring. The key extends radially outward from the base 16 of the inner ring groove and is preferably radiused at the base to merge with the walls of the groove base, this being accomplished during the machining of the groove. The key protrudes downwardly from the upper flange 13, and its outer periphery is an extension of the periphery of the flange 13, so that the key is readily visible when the ring member is viewed from its outer edge as illustrated in FIG. 2.

To permit the joint of the outer ring member 12 to close, in assembled relation, the ends of the outer ring are provided with notches 31 and 32 to accommodate the key 30; and these notches then coact with the key to maintain the desired rotational relation between the inner and outer rings wherein the respective joints are disposed diametrically opposite from each other. As best seen in FIGS. 2 and 5, the notches 31 and 32 are provided in the upper flange 20 of the outer ring, with the notch 32 being formed in the portion of this flange which is a part of the tongue 26a.

As indicated above, the key 30 is readily visible from the exterior edge of either the unassembled inner ring member 11 for the assembled ring members 1 1 and 12. This is of value during the assembly of the ring members, in that the assembler may easily orient the two ring members and assemble them without damaging the key 30 and thereby destroying the antirotation feature.

Embodiment of FIGS. 8 and 9 FIGS. 8 and 9 illustrate a compression ring assembly 1 similar to that of FIGS. 1 through 7 but including an additional member in the form of an expansion ring 35. The interlocking ring members of this assembly are substantially identical to those of FIG. 1; and therefore the same reference numbers will be used to identify the ring members 11 and 12 and associated parts.

The expansion ring 35 consists of a washer-like ring of suitable spring metal having a main body 36 in the form of an annular disc and an inner cylindrical flange 37. In assembled relation, the body portion 36 overlies the upper flange 13 of the inner ring 11; and the upper portion of the inner cylindrical wall of the ring member 11 is recessed to define an annular groove for accommodating the cylindrical flange 37 of the expansion ring.

As best seen in FIG. 8, the body portion 36 is provided with angularly spaced radial slots 38 extending inwardly from the outeredge of the expansion ring about one-half the radial width of the ring; and the body portion therefore defines outward extending radial fingers 39. Preferably the slots 38 terminate in small drilled or punched holes to prevent breakage.

The cylindrical flange 37 of the expansion ring is provided with angularly spaced slots 41 which extend longitudinally the full width of the cylindrical flange and also extend radiallya short distance outward from the 'inner periphery of the body portion 37. The slots 41 also preferably terminate in small holes.

The slots 38 and 41 provide great flexibility for the expansion ring 35,'permitting the expansion ring to seat itself radially on the compression ring assembly and obviating working of the expansion ring.

FIG. 9 illustrates two piston ring assemblies 10 and 34 in combination with'the grooves ofa piston 43. The upper assembly 34, as seen in FIG. 9, is an assembly interlocking compression rings 11 and 12 and an expansion ring 35; while the lower assembly 10 includes only the interlocking compression rings 11 and 12. For either assembly, the normal stack-up of ring assembly members within a piston groove would provide for a clearance of approximately 0.001 inch; and this clearance is of course greatly exaggerated in FIG. 9.

Referring particularly to the assembly 34 including the expansion ring 35, the fingers 39 are shown as being curved upwardlyfrom the perpendicular plane of the inner portion of the expansion ring body 36 and of the upper surface of the compression ring flange 13. These fingers then bear against the upper wall of the piston groove whereby the expansion ring urges the assembly of the ring members 11 and 1 2 downwardly against the lower wall of the piston groove. The expansion ring is dimensioned to assure that the tips of the fingers 39 do not extend out of the piston groove into engagement with the cylinder wall. The expansion ring 35 then obviates hammering of the compression ring assembly between the walls of the piston groove. In an engine application, wherein the top of the piston is indicated in FIG. 9, the compressed gaseswill assist the expansion ring in maintaining the compression ring assembly against the lower wall of the groove.

Embodiment of FIGS. 10 through A compression. oil ring assembly 50 is illustrated in FIGS. 10 through 15, made up of interlocking ring members consisting of an outer compression ring member 51 and an inner oil ring member 52. As best seen in the radial sectional view of FIG. 14, the outer compression ring includesupper and lower radial flanges 53 and 54 joined by an interconnecting cylindrical web 55 which defines the outer cylindrical wall of the ring member for engagement with a cylinder wall. This ring member then defines an inward facing annular groove having a cylindrical base 56. The inner oil ring 52 includes upper and lower flanges 57 and 58, respectively, interconnected by a cylindrical web 59; this member defining an outward facing annular groove having a cylindrical base 60. The lower flange 54 of the-compression ring and the upper flange 57 of the oil ring define interlock flanges received in the grooves of the opposite members, with the peripheral cylindrical walls of these flanges seating in the respective grooves to provide the desired sealing. The flange 57 is fully received within the mating groove in the manner described with reference to the embodiment of FIGS. 1 through 7; however, the groove of the oil ring 52 has a width substantially greater than that of the interlock flange 54 so that the ring assembly 50 defines an outward facing'annular oil groove 61. The web 59 of the oil ring is provided with angularly spaced slots 62, in the plane of the oil groove 61, to provide for radial communication between the oil groove and the piston groove within which the ring assembly 50 is contained.

The lower face of the compression ring flange 54 is provided with an annular, peripheral recess defining a wiper edge 63 for wiping oil from the cylinder wall. On the downstroke of the piston, the wiper lip 63 wipes the oil from the cylinder walls, the oil being collected in the oil groove 61.

When the ring assembly 50 is assembled in a piston cylinder assembly the inner oil ring 52 is urged outwardly against the outer compression ring 51 to effect the seating of the cylindrical edges of the interlocking flanges with the respective cylindrical groove bases 56 and 60. The axial relationship of the ring members is determined by the upper interlock flange 57 of the oil ring which is received within the inward facing groove of the compression ring, with the confronting beveled surfaces of the interlocking flanges being provided with an approximately 10 bevel in the same manner as the ring assembly 10 of FIGS. 1 through 7. Similarly, a clearance 64 of about 0.0002 inch is provided between the confronting beveled surfaces in the manner of the assembly 10 and for the same reasons.

. In this combination compression-oil ring assembly 50, it is only the outer compression spring 51 which engages the walls of the engine cylinder, the lower flange I 58 of the oil ring having a lesser outer diameter to provide an annular clearance space of about 0.025 inch between this oil ring flange and the cylinder wall. With this arrangement, on the upstroke of the piston, oil which has been collected in the oil ring 61 is deposited on the cylinder wall by the lip of the flange 58 which includes an upper bevel 69 to'assist in flowing the oil from the oil groove.

FIG. 13 of the drawing is a perspective view of the joint for the outer compression ring viewed in the lower portion of FIG. 10. As best seen in FIG. 13, the ends of the compression ring 51 are provided with interengaging tongues '65 and 66 which coact with each other and with the interlock flange 57 of the oil ring to provide a seal both radially and longitudinally of the piston with which the ring is assembled. The tongue 65 is formed principally at the outer periphery of the compression ring, at one of the inter-engaging ends while the tongue 66 is former principally at the inner periphery of the ring. The tongues are arranged to overlap both radially and longitudinally; and for this purpose each of the overlapping tongues is of a thickness radially equal to about one-half of the radial thickness of the upper flange 53, and of a thickness transversely equal to about one-half of the depth of the Web 55- in a direction longitudinally of the piston. These tongues fit into and slide in corresponding grooves in the mating ends of the compression ring. I

The portion of the tongue 66 formed in the flange 53 is stepped radially to define an extension 66a.'A verti-' cal cylindrical surface 66b of the step lies generally parallel to the outer cylindrical surface of the oil ring interlock flange 57 and is radially spaced therefrom to obviate a direct longitudinal path through the flanges 53 and 57, and thereby obviate longitudinal blow-by of the compressed gases. A portion of the tongue 65 formed in the web 55 is provided with a longitudinal extension 650. A horizontal surface 65b defined by the extension 65a is spaced longitudinally from the upper flat surface of the interlock flange 57 to thereby obviate a direct radial path between the outer and inner edges of the ring assembly along the flange 57.

When the ring assembly 50 is compressed in a piston cylinder assembly the joint is closed in the manner indicated in FIG. 12.

FIG. 15 illustrates in detail the joint between the ends of the oil ring members 52 which is indicated at the top of FIG. 10. For this joint, the oil ring is split generally along a plane passing through the ring axisv Embodiment of FIG. 16

A compression-oil ring assembly 50a is illustrated in FIG. 16 made up of interlocking ring members consisting of an outer compression ring member 51a and an inner oil ring member 52a. This ring assembly 50a is generally similar to the assembly 50, with the oil ring member 520 being identical to the oil ring member 52 and with thecompression ring member 51a differing from the compression ring 51 only in the configuration of the outer peripheral sealing surface defined by the web 55a. The identical parts of the ring members and assembly are identified by the same reference numbers.

The outer peripheral face-of the compression ring member 51a is defined by a cylindrical wall portion 71 adjacent to the wiper lip 63, which portion of course seals with the cylinder wall, and a frusto-conical wall portion 72 which tapers radially inwardly from the cylindrical portion 71 to the outer face of the compression member upper flange 53.

The taper of the frusto-conical wall portion is preferably about l to 2 relative to the cylindrical wall portion 71, this taper being greatly exaggerated in the drawing for purposes of illustration. The axial depth of the cylindrical portion 71 may be for example about one third of the total depth of the peripheral face of the compression ring member or about one-fifth or onesixth of the total depth of the entire ring assembly. Where a typical compression-oil ring assembly may have an overall depth or thickness of three-sixteenths inch for example, the depth of the cylindrical ring surface 71 may be one thirty-second inch.

A compression-oil ring assembly having this configuration will seat more rapidly on a cylinder wall when newly inserted in an engine because of the smaller cylindrical surface area; and will therefore function at maximum efficiency much more rapidly. The taper of the frusto-conical portion should be at a very small angle relative to the cylindrical portion so that the ring member 51a will tend to glide over any remaining oil film on the cylinder wall as the piston moves toward top dead center. If the angle between the frusto-conical portion and the cylindrical portion is too great, in excess of 3 for example, the junction of these surfaces will present a ridge which tends to wipe any oil film on the cylinder wall rather than glide over the oil film as desired. 1

What has been described are several forms of piston ring assemblies according to the invention, each assem- 8 bly including two interlocking rings having an improved interlock structure to provide better sealing between the ring members to prevent blow-by of the compressed gases both radially and longitudinally of the piston on which the ring assembly is mounted.

A feature of the invention is an improved combination compression ring-oil ring assembly including the above mentioned improved interlock seal feature and further including improved means for depositing oil on the cylinder wall on the upstroke of the piston.

Another feature of the invention is the provision of the designed clearance between the confronting beveled faces of the interlock flanges to maintain the desired sealing between the interlocking rings over the expected life of the assembly, while accommodating wear of the ring members.

Still another feature of the invention is the provision of an improved expander ring for urging and maintaining the remainder of the assembly in engagement with the cylinder walls, even though the cylinder walls may assume a slightly out-of-round condition during operation of the engine.

While preferred embodiments of the invention have been illustrated and described, it will be understood by those skilled in the art that changes and modifications may be resorted to without departing from the spirit and scope of the invention.

What is claimed is:

1. A piston ring assembly comprising upper and lower interlocking split rings, each ring having upper and lower radial flanges and an interconnecting web defining an annular groove; the ends of said upper ring having interlocking radially and axially overlapping fingers;

said upper ring annular groove facing inward, with its web defining the outer ring wall; said upper ring annular groove being dimensioned to receive the lower ring upper flange in interlocking sealed relation, whereby said lower ring is radially confined by said upper ring;

said lower ring annular groove facing outward, and having sufficient width to accommodate the upper ring lower flange and to define an oil groove for said assembly between the confronting faces of the two lower flanges;

in assembly, said lower ring lower flange having an outer diameter less than the upper ring outer diameter whereby said lower ring lower flange is maintained in slightly spaced relation to the cylinder wall.

2. A piston ring assembly as set forth in claim 1 a lower face of said ,upper ring being provided with an annular recess adjacent to its outer periphery to define an acute annular wiping edge for wiping the cylinder wall.

3. A piston ring assembly as set forth in claim 1 the web of said lower ring member having transverse ports for communicating said oil groove with the base of the piston groove containing said ring as sembly.

4. A piston ring as set forth in claim 1 v the outer diameters of said upper ring and of said lower ring lower flange being dimensioned to provide, in assembly, a clearance of about 0.025 inch between said lower flange and the cylinder wall; said lower flange defining anoil spreader ring for depositing oil from the oil groove onto the cylinder 1 wall during the upstroke of the piston.

5. A piston ring as set forth in claim 1 said lower ring lower flange having an outer peripheral bevel between its outer and upper faces.

6. A piston ring as set forth in claim 1 the outer peripheral wall of said upper ring comprising a lower cylindrical portion for engagement with the cylinder wall and a merging upper, inwardly diverging tapered portion; said tapered portion being inclined at a very slight angle relative to said cylindrical portion.

7. A piston ring assembly as set forth in claim 6 said tapered portion being inclined about 1 relative to said cylindrical portion.

8. A piston ring assembly as set forth in claim 1 including a split expansion ring of spring metal comprising a flat annular portion, disposed to engage one flat face of the interlocking ring assembly, and a cylindrical flange disposed to engage the inner cylindrical wall of the interlocking ring assembly;

said flat portion providing angularly spaced, outward extending, radial fingers; said fingers being curved in a direction opposite from said cylindrical flange to bear against a piston groove wall;

and said cylindrical flange being slotted to define angularly spaced segments, said expansion ring being radially flexible.

9. A piston ring assembly as set forth in claim 8 wherein said cylindricalflange has angularly spaced longitudinal slots extending into the flat annular portion of the expansion ring to define said angularly spaced segments.

10. A piston ring assembly as set forth in claim 1 said upper ring lower flange, said lower ring upper flange, and the groove bases having respective confronting cylindrical seal surfaces seating on each other in assembled relation; and said flanges having confronting annular seal surfaces beveled at an angle of approximately 10 relative to the plane of the ring assembly; and said ring members being dimensioned to provide a clearance of approximately 0.002 inch between the confronting beveled surfaces when'said ring members are compressed in assembled relation. 

1. A piston ring assembly comprising upper and lower interlocking split rings, each ring having upper and lower radial flanges and an interconnecting web defining an annular groove; the ends of said upper ring having interlocking radially and axially overlapping fingers; said upper ring annular groove facing inward, with its web defining the outer ring wall; said upper ring annular groove being dimensioned to receive the lower ring upper flange in interlocking sealed relation, whereby said lower ring is radially confined by said upper ring; said lower ring annular groove facing outward, and having sufficient width to accommodate the upper ring lower flange and to define an oil groove for said assembly between the confronting faces of the two lower flanges; in assembly, said lower ring lower flange having an outer diameter less than the upper ring outer diameter whereby said lower ring lower flange is maintained in slightly spaced relation to the cylinder wall.
 2. A piston ring assembly as set forth in claim 1 a lower face of said upper ring being provided with an annular recess adjacent to its outer peripherY to define an acute annular wiping edge for wiping the cylinder wall.
 3. A piston ring assembly as set forth in claim 1 the web of said lower ring member having transverse ports for communicating said oil groove with the base of the piston groove containing said ring assembly.
 4. A piston ring as set forth in claim 1 the outer diameters of said upper ring and of said lower ring lower flange being dimensioned to provide, in assembly, a clearance of about 0.025 inch between said lower flange and the cylinder wall; said lower flange defining an oil spreader ring for depositing oil from the oil groove onto the cylinder wall during the upstroke of the piston.
 5. A piston ring as set forth in claim 1 said lower ring lower flange having an outer peripheral bevel between its outer and upper faces.
 6. A piston ring as set forth in claim 1 the outer peripheral wall of said upper ring comprising a lower cylindrical portion for engagement with the cylinder wall and a merging upper, inwardly diverging tapered portion; said tapered portion being inclined at a very slight angle relative to said cylindrical portion.
 7. A piston ring assembly as set forth in claim 6 said tapered portion being inclined about 1* relative to said cylindrical portion.
 8. A piston ring assembly as set forth in claim 1 including a split expansion ring of spring metal comprising a flat annular portion, disposed to engage one flat face of the interlocking ring assembly, and a cylindrical flange disposed to engage the inner cylindrical wall of the interlocking ring assembly; said flat portion providing angularly spaced, outward extending, radial fingers; said fingers being curved in a direction opposite from said cylindrical flange to bear against a piston groove wall; and said cylindrical flange being slotted to define angularly spaced segments, said expansion ring being radially flexible.
 9. A piston ring assembly as set forth in claim 8 wherein said cylindrical flange has angularly spaced longitudinal slots extending into the flat annular portion of the expansion ring to define said angularly spaced segments.
 10. A piston ring assembly as set forth in claim 1 said upper ring lower flange, said lower ring upper flange, and the groove bases having respective confronting cylindrical seal surfaces seating on each other in assembled relation; and said flanges having confronting annular seal surfaces beveled at an angle of approximately 10* relative to the plane of the ring assembly; and said ring members being dimensioned to provide a clearance of approximately 0.002 inch between the confronting beveled surfaces when said ring members are compressed in assembled relation. 